Air dispensing apparatus, that is apparatus enabling the inflation of tires from a compressed air supply, is commonly provided at petrol stations, motor service stations, municipal bus garages etc. and it is well known that the inflation of tires to the correct pressure is a necessity for safety reasons and for fuel economy and for optimizing the life of the tires. Regrettably the apparatus hitherto available for this purpose has, in general, relied upon measurement of the tire air pressure by a simple mechanical pressure gauge, in some cases pre-set to a required pressure at which inflation is to be terminated and in other cases arranged merely to display to the user the pressure of air which has been introduced into the tire. It is found that while arrangements of this kind may be set up with reasonable accuracy upon first installation, the maintenance of appropriate accuracy during prolonged periods of use is a matter of great difficulty. Frequent recalibration of the pressure gauge mechanism would be required if such mechanical arrangements were to be expected to function accurately for long periods and unfortunately it is the case that such recalibration either is never effected or is effected only at such long intervals that substantial inaccuracies arise.